In the past, conventional processes for pulping fibrous raw materials utilized sulfur-containing compounds. Such compounds were used because of their excellent lignin removing activity. However, waste and spent liquors from such processes caused water and air pollution problems, primarily due to the presence of such sulfur-containing compounds. Accordingly, attempts have been made to accomplish pulping using mechanical or chemical processes, or a combination of the two, without resorting to the use of sulfur-containing compounds.
For example, Worster et al, U.S. Pat. No. 3,691,008, discloses a two-stage pulping process wherein wood chips are subjected to a mild digestion process using sodium hydroxide, mechanically refined, and then subjected to a second pulping-digestion process in the presence of sodium hydroxide and oxygen. Others have substituted various other alkalis for sodium hydroxide such as sodium carbonate, sodium bicarbonate, ammonia, sodium tetraborate, sodium benzoate, magnesium oxide, magnesium hydroxide, and magnesium carbonate.
Thus, Nagano et al, U.S. Pat. No. 4,045,279, uses sodium carbonate and bicarbonate in a pulping process which pre-cooks wood fibers in an aqueous alkaline solution, defibrates the material, and then delignifies the defibrated material in the presence of oxygen and a solution of sodium carbonate and bicarbonate. Canadian Pat. No. 982,759, utilizes a process quite similar to Nagano et al in pre-treating wood fibers with an aqueous solution of alkali, defibrating the material by refining, and pulping the defibrated material in the presence of oxygen and alkali. Finally, Markham, "Use of Borax in Oxygen Pulping", given at the Canadian Pulp and Paper Association Technical Section annual meeting Feb. 1, 1978, noted several advantages of using sodium tetraborate (borax) over sodium carbonate in a single stage oxygen pulping process. Others have discovered that use of high ionic strength borate compounds in oxygen pulping processes increases both delignification rates and pulping selectivity (i.e., yield at a given lignin content).
However, none of the above-mentioned prior art has addressed the problem of obtaining high quality pulp from an oxygen pulping process and simultaneously obtaining a spent liquor which is economically recoverable. It has been found that refining and delignification of high consistency fibrous material (i.e., having a liquor to wood ratio of 4/1 to 6/1) results in pulp having poor qualities such as low strength and brightness. By using a low consistency fibrous material (i.e., having a liquor to wood ratio of 12/1 to 30/1), a more homogeneously delignified pulp is obtained with good strength and brightness properties. Still, use of low consistency fibrous material in refining and oxygen pulping has heretofore been uneconomical because of the high cost of recovery of such dilute spent liquor. In addition, Marton, Empire State Pulp Research Association Report No. 60, Mar. 15, 1974, proposed to oxygen delignify low consistency fibrous material with sodium hydroxide containing liquor and recycle that liquor to the pulping stage of the process, but determined that such recycling was not feasible because of poor quality pulp obtained from such a process. Marton used only a 20% strength sodium hydroxide liquor.
More recently, Nagano et al, U.S. Pat. No. 4,089,737 suggests using a relatively low consistency (liquor to wood ratio of from 5/1 to 20/1) pulp in carrying out an oxidative delignification of pulped material as a first stage of a bleaching process with recycle of liquor. A similar disclosure is found in "Oxygen Bleaching: Low-Consistency Oxygen Delignification System Uses Continuous Pipeline Reactor," Paper Trade Journal, July 15-31, 1978, pp. 37-39. However, there is no teaching of a method to obtain simultaneously pulp of high strength properties and an economically recoverable spent liquor while performing the majority of the delignification in the oxygen stage rather than in a prior delignification stage. Accordingly, the need exists in the art for an economical oxygen delignification process which yields high quality pulp.